The invention relates to metering apparatus in general, and more particularly to improvements in apparatus which can be used to meter flowable materials in sand core making machines, especially for metering aggressive liquid bonding agents in such machines.
It is customary to transfer metered quantities of a liquid bonding agent from a source of such medium into a mixing unit. The arrangement is normally such that the bonding agent is transferred by pressurization and/or under the action of gravity. As a rule, the means for transferring metered quantities of bonding agent to the mixing unit includes a diaphragm pump or a piston pump. The metering action of a diaphragm pump is determined by appropriate selection of the frequency and extent of deformation of the diaphragm and is a function of the length of the interval of operation of the thus adjusted diaphragm pump. The adjustment is simpler in connection with a piston type pump because the quantity of displaced bonding agent is directly dependent on the number of strokes of the pump piston. A piston pump which is used in conventional metering apparatus is disclosed in German Offenlegungsschrift No. 2 262 254 of Michel. The bonding agent comes in direct contact with the internal surface of the cylinder and with the piston of the pump.
The just outlined metering apparatus exhibit a number of serious drawbacks when they are called upon to deliver metered quantities of customarily employed bonding agents for sand in sand core making machines of foundries or like plants. Thus, a diaphragm pump is simply incapable of metering a bonding agent with a requisite degree of accuracy, primarily or at least in part because the seals in such pumps are rapidly damaged or totally destroyed by the often highly aggressive bonding agent. The situation is analogous when the metering means is a piston pump, i.e., the bonding agent rapidly damages or destroys the seals. Moreover, the bonding agent is likely to affect the condition of surfaces in the regions of sliding contact between the piston or pistons and the associated cylinder or cylinders (not the Michel reference). The resulting leaks allow for penetration of air which further affects the accuracy of the metering action. Furthermore, and even if a pump operates properly, it is highly unlikely to deliver, for a reasonably long period of time, metered quantities of bonding agent which comes into direct contact with its parts because and, even minute, leaks in the suction pipe which connects the source of bonding agent with the pump will immediately affect the accuracy of the metering action because air can be drawn into the pump chamber to thus effect a reduction of the quantity of bonding agent which is withdrawn from the source. The remedial action necessitates a lengthy interruption of the operation of the pump in order to evacuate air from the pump chamber and/or to replace or repair the suction pipe.